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Swine flu — influenza A(H1N1) — has been circulating undetected for some time, scientists reported in Science [557kB] last week (22 May).

The international collaboration found that A(H1N1) is made up of genes from flu viruses seen in birds, humans and pigs — and parts of its genetic make-up date back to the 1918 influenza pandemic.

The extensive differences between the genes of A(H1N1) and its nearest relatives suggest it has been circulating for an "extended period" — perhaps because of poor surveillance of swine herds, say the researchers. And the absence of the known characteristics that make viruses capable of spreading between humans suggests that A(H1N1) has previously unrecognised features.

Meanwhile, scientists in Singapore have created an interactive three-dimensional model of A(H1N1)'s neuraminidase protein — a virus surface protein. The model reveals that although the protein is different to that in other flu strains it is still vulnerable to drugs such as oseltamivir (Tamiflu).

The WHO is considering including the lethality of the A(H1N1) virus in criteria for deciding whether the pandemic alert level should be raised from 5 to 6, Keiji Fukuda, WHO assistant director-general for health security and environment said in a WHO press briefing [110kB] last week (22 May). The current system includes information only about the spread of the virus.

Reuters reported that the World Health Assembly also agreed to delay decisions on how the world would share virus samples, vaccines and drugs in the event of a pandemic, tasking WHO director-general Margaret Chan with finalising the plans by early 2010.

Sticking points in the talks — which have been in progress since 2007 — include the content of a 'material transfer agreement', in which developing countries want agreements that they will have access to enough affordable vaccine.

Last week's (22 May) WHO weekly epidemiological report [1.42MB] provides an analysis of the clinical picture of A(H1N1) infection and how best to treat it. The report says pre-existing conditions such as asthma and diabetes make patients more vulnerable.

Symptoms are the usual signs of flu with occasional vomiting and diarrhoea. Hardest hit are children under nine but they are generally the quickest to recover.

The US Centers for Disease Control's Morbidity and Mortality Weekly Report reported last week (22 May) that some elderly people in the United States may have a degree of pre-existing immunity to A(H1N1), possibly because they have been exposed to a similar virus before — through vaccination or infection.

And contrary to other reports, Stanford University scientists have found that face masks can prevent transmission of influenza. They produced a mathematical model in which face masks were 98 per cent effective in stopping transmission. This is reduced to 30 per cent once someone starts showing symptoms.

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